Pumps of this type are known. They have an adjusting device for adjusting the stroke ring, wherein the adjusting device has at least one pressure-action surface for generating an adjusting force upon the stroke ring, which adjusting force, regulated for instance by a valve, is generated by an adjusting pressure which lies between the high pressure and the suction pressure of the pump (so-called feed control).
Other known pumps have an adjusting device comprising a pressure-action surface for generating an adjusting force upon the stroke ring and a further, additional pressure-action surface for generating a compensation force. The adjusting force (generally jointly with an additional spring for generating a spring force), by virtue of a pressure controlled by the valve, will here adjust the stroke ring in the direction of high eccentricity (full stroke), while the pressure-action surface for the compensation force as the counter-force (in the prior art) is directly connected to the pressure region of the pump (so-called discharge control).
Known pumps of this type have problems. Thus, in the region of the pump between the suction region and the high-pressure region is arranged a separation region of at least one cell width, which is intended to prevent a short-circuit between the pressure region and the suction region. The cells rushing through this separation region are thus transported from the suction region with suction pressure into the pressure region and are there charged by the high pressure which prevails there. During this passage through the separation region, alternating pressure fields and pressure fluctuations, which impact as internal forces on the stroke ring pressure-clamped by the external and internal pressures, are thus constantly produced by the so-called reversal of the rapidly passing cells in the peripheral direction.
Furthermore, in the case of different eccentric positions of the stroke ring, different angular positions of the separation region relative to the stroke ring also arise.
In the separation region, moreover, at full stroke, as a result of a geometrically based precompression in the closed-off cell region, a pressure which can be higher than the system pressure, momentarily set by a consuming unit, in the pressure region of the consuming unit, for instance a transmission, can optionally prevail. At zero delivery, no precompression prevails in the separation region.
In addition, the high pressure is dependent on the momentary operating states of the consuming unit (for example, the transmission) and is thus subject to large fluctuations.
The adjusting force upon the stroke ring derives from an equilibrium of the forces which are generated by the pressure-action surface of the control chamber, by the pressure-action surface of the compensation chamber, by the spring and by the force vectors defined by the position of these chambers. It is opposed by the inner forces in the stroke ring which result from the cells loaded with system pressure or suction pressure, which cells constantly change, as well as by friction forces.
Since the resultant force vector of the system-based forces within the pump thus varies in size and direction with different operating states, the control pressure, i.e. pressure in the control chamber, must be changed correspondingly in order to achieve a force equilibrium. This leads in the prior art to persistent control deviations.
The pressure distribution within the movably mounted component, i.e. the stroke ring, thus has a substantial influence on the changes in the pump-internal system-based forces. In particular at the places at which passing cells change from suction pressure to system pressure or vice versa, fluctuating forces, which exert very different loads in dependence on the operating point, are active.